If you move the walls of a gas-filled box to expand its volume, and do it (arbitrarily) slowly enough that the gas remains (arbitrarily) close to equilibrium throughout, then the expansion is isentropic and reversible.

If we restrict such a universe to contain _only_ radiation, then its expansion and re-contraction are basically reversible and isentropic (I argued this back in the “Latest Declamations About the Arrow of Time” post, and you more-or-less agreed; see comments #60, 61 and 67 in that thread).

The working fluid is air, there is no friction, the compression and expansion are isentropic, the air temperature does not change during the heat addition and heat rejection, the air behaves as an ideal gas (i.e., pv = RT) and the specific heat is constant.